The present invention relates to a vacuum brake power booster including a booster housing which is sealingly subdivided by an axially movable wall into a vacuum chamber and a power chamber. The axially moveable wall is subjectible to a pneumatic pressure differential. A control valve, which controls the pressure differential, is accommodated within a control housing and actuatable by a valve piston. The valve piston is slidable by means of a piston rod and in a force-transmitting connection with a force-delivering member, such as a push rod, through an elastic reaction disc. The valve piston is composed of a first piston part which interacts with the piston rod and is rotatably supported within the control housing, and a second piston part which is guided within the control housing so as to be secured against rotation and is in abutment against the reaction disc. The two piston parts are coupled to each other by means of a threaded union so as to be movable in respect of each other within limits in an axial direction. A tubular sleeve, which is arranged coaxially to the piston rod, is in engagement with the first piston part.
In series manufacture of vacuum brake power boosters of this type, it is necessary to adjust the distance between the reaction disc and the second piston part as exactly as possible. The distance is typically referred to as the parameter z. The exact adjustment is needed in order to obtain the characteristic curves required by the automobile industry. The parameter z to be adjusted determines the behavior of the booster in the initial phase of its actuation. In particular the extent of the abrupt rise of the output force when actuated with a predetermined input force (i.e., the jumper effect) depends on the parameter z.
In an adjustable vacuum brake power booster as disclosed in DE-OS 42 08 384, the adjustment of the desired parameter z is carried out by means of a sleeve, which is arranged coaxially to the piston rod and is in permanent engagement with the first piston part. The sleeve is secured against rotation due to its positive engagement with the control housing after the adjusting operation is terminated. The anti-rotation mechanism is preferably effected by a circlip which is slipped onto the sleeve and the radial projections of which engage into grooves provided on the inner circumference of the control housing. Subsequently, the circlip is retained by the fringe area of an elastic protective cap which protects the control housing, which fringe area is coupled to a circumferential groove in the sleeve.
The disadvantage in the above referenced adjustable vacuum brake power booster is that variation of the adjusted parameter z within the automotive vehicle is not possible.
Therefore, an object of the present invention is to make provisions in a vacuum brake power booster of the type previously mentioned which allow infinitely variably adjusting the adjusted parameter z when the booster is in the completely assembled condition and fitted to the automotive vehicle.
This object is achieved, according to the present invention, by the sleeve having an extension at its end projecting from the control housing which allows rotation of the sleeve.
Preferred embodiments of the present invention are described in the following description of two preferred embodiments making reference to the accompanying drawings.